1. Field of the Invention
The present invention relates to improvements in vapor-cell atomic frequency standards. More particularly, this invention pertains to an improved microwave cavity resonator for use in such systems.
2. Description of the Prior Art
Atomic frequency standard systems are commonly utilized to regulate the frequency of quartz crystal oscillators that otherwise lack frequency stability. Essentially, such systems attempt to lock the frequency of the quartz crystal oscillator onto the natural frequency of oscillation of an element such as an alkali metal. The frequency associated with the element corresponds to the difference in energy between the two hyperfine structure levels of the ground state of the atom which, for the isotope .sup.87 Rb, lies at about 6.83 GHz.
The mechanism for regulating the quartz oscillator is an atomic resonance system, which generally includes a gas discharge lamp filled with the desired alkali metal vapor (e.g. rubidium) that is optically aligned with a cell consisting of an evacuated glass bulb filled with the vaporized alkali metal and with a photosensitive light receiver positioned to detect light emanating from the lamp after passage through the cell. Such a system is disclosed, for example, in U.S. Pat. No. 3,798,565 of Jechart for "Gas Cell Atomic Frequency Standard of Compact Design".
In accordance with such a system, the light spectrum of the selected element radiated from the lamp (which, incidentally, is actuated to discharge by r.f. energy) is absorbed by the rubidium in the absorption cell in a process known as resonance absorption or "optical pumping" in which the equilibrium populations of the two hyperfine ground states are preferentially altered. This process reduces the light absorption capacity of the vapor within the cell so that, after a period of time, an increase in the intensity of the light striking the photodetector after passage through the cell may be detected.
In an atomic frequency standard, a type of closed-loop control system, the effect of the optical pumping process upon the light absorption properties of the element within the cell are counteracted and the "error signal" resulting therefrom driven to zero by the injection of electromagnetic energy of frequency equal to that of the atomic resonance of the element within the absorption cell. A microwave cavity is commonly provided in an atomic frequency standard system for coupling the injected electromagnetic energy to the atoms of the vapor within the cell. The microwave cavity (resonator) is designed for resonance at the atomic frequency of the element to assure efficient injection of the electromagnetic energy, which energy is derived by frequency multiplication of the output of the monitored quartz oscillator. When the derived frequency of the injected electromagnetic energy is precisely equal to the atomic frequency of the element, the effect of the optical pumping process is reversed, and the light absorption of the element within the cell increased significantly, an effect that is detected by the photosensitive element. A feedback system, coupled to the resonator and to the photodetector, then utilizes the detected change in the absorption property to monitor and drive the frequency of the quartz crystal oscillator to and maintain it at its preselected nominal value.
The effective functioning of an atomic frequency standard in accordance with the general configuration and mode of operation as above described makes numerous demands upon the system's microwave cavity resonator. As mentioned, it must be accurately tuned to the resonant frequency of the alkali metal for efficient injection of electromagnetic energy of the desired frequency therein. Additionally, it should preferably enhance the interaction of the injected electromagnetic energy with the cell and its contents. In the past, cylindrical cavity resonators have been designed to support the TE.sub.011 and TE.sub.111 modes. The former designs (TE.sub.011) generally provide efficient coupling to the absorption cell. However, the designs of cavities have been unacceptably large for use in airborne or satellite environments. For example, TE.sub.011 mode cylindrical cavities have required, at a minimum, a diameter of approximately 2.5 inches and length of approximately one inch to function effectively. The TE.sub.111 mode cavity can be designed for lesser size; a cavity of about one inch diameter and length will support this mode. However, due to the nature of the TE.sub.111 standing wave, the electromagnetic energy injected into the cavity is not coupled efficiently into the internal absorption cell as the magnetic flux of the injected energy is concentrated about the edges of the cavity.